1. Field of the Invention
This invention relates generally to a sheet material counting apparatus, and particularly to an optical detection device for use in a bank note counting apparatus wherein the influence of dust or change in temperature is excluded to maintain the quantity of light received by the light receiving element at a constant level.
2. Prior Art
There has been known in the art a device for detecting a passing bank note by the provision of a light emitting element arranged along the bank note passage and a light receiving element disposed opposite the light emitting element to detect the passing bank note by sensing the change in the output from the light emitting element.
However, in a known device of this type, the quantity of light incident on the light receiving element might be reduced by the adhesion of dust to the light emitting or receiving element of by change in characteristic features of the light emitting or receiving element due to temperature change. As a result, the detection level of the receiving element might be shifted, leading to erroneous operation of the device.
In order to obviate this erroneous operation due to reduction in the quantity of light incident on the light receiving element, there has been proposed a device wherein the quantity of light received by the light receiving element (the quantity of light being converted into the light receipt output voltage), when no bank note interrupts the path of light from the light emitting element to the light receiving element, is sample-held as the standard quantity of light (standard light receipt output voltage); and the divided value of this standard voltage is compared to the quantity of light (which is converted into an output voltage) sensed at the passage of bank note to thereby know the presence or absence of bank note.
Another improvement was proposed by the Japanese Patent Application No. 48806/1976. The improved device proposed by the application comprises a detector for detecting the reduction in quantity of light sensed by the light receiving element, an amplifier for compensating the reduction in quantity of light, and means for automatically increasing the gain of the amplifier based on the output from the detector thereby to retain the electrical detection level at a constant value.
However, these improved devices, which have been previously proposed, have a common disadvantage that the output voltage from the light receiving element cannot be differentiated from the noise voltage when the quantity of light sensed by the light receiving element is decreased extremely, for example, due to dust or paper scraps adhering to either the light emitting or receiving element, making it difficult to conduct the comparison and discrimination operations.